Subsoil moisture monitoring system including battery-less wireless chipless sensors

ABSTRACT

A batteryless, chipless, sensor is disclosed which includes a substrate, at least two conductive strips disposed on the substrate, a passivation layer encasing the substrate and the at least two conductive strips, wherein the conductive strips are adapted to respond to an interrogation signal from a reader having a first polarization, with a response signal at a second polarization different than the first polarization.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to and claims the priority benefit of U.S.Provisional Patent Application having Ser. No. 63/105,456 titled“BATTERY-LESS WIRELESS CHIPLESS SENSORS FOR SUBSOIL MOISTURE MONITORING”which was filed Oct. 26, 2020, and U.S. Provisional Patent Applicationhaving Ser. No. 63/158,398 titled “SUBSOIL MOISTURE MONITORING SYSTEMINCLUDING BATTERY-LESS WIRELESS CHIPLESS SENSORS” which was filed Mar.9, 2021, the contents of each of which are hereby incorporated byreference in their entirety into the present disclosure.

STATEMENT REGARDING GOVERNMENT FUNDING

None.

TECHNICAL FIELD

The present disclosure generally relates to systems for monitoringsubsoil conditions, and in particular, to a subsoil moisture monitoringsystem including battery-less wireless chipless sensors.

BACKGROUND

This section introduces aspects that may help facilitate a betterunderstanding of the disclosure. Accordingly, these statements are to beread in this light and are not to be understood as admissions about whatis or is not prior art.

The world population is predicted to reach 9.8 billion in 2050 and thefood demand is estimated to increase by 59-98% between 2005 and 2050.Therefore, food productivity is an important concern in the increasingdemand for sustainable agriculture. However, inattentive efforts toimprove food productivity, such as unessential usage of fertilizers,have led to adverse ecological conditions including the disruption ofthe natural nitrogen cycle and degradation of soil quality. Precisionagriculture focuses on the careful management of agricultural resourcesthrough real-time soil health monitoring at a large scale. As fooddemand increases, optimizing current methods of production and findingnew sustainable ones becomes vitally important. Soil health monitoringfor precision agriculture requires large scale deployment of sensors tomonitor soil parameters such as moisture, temperature, microbialactivity, nitrogen concentration etc., and to report the status of soilto farmers. Among various parameters, moisture, commonly quantified bythe volumetric water content (VWC), is an important parameter inagriculture due to its influence in plant growth, nutrient transport,and soil properties. Changes in VWC have also been found to directlyaffect microbial biomass and enzymatic activity of microorganisms indifferent soil types. Microorganisms perform crucial tasks in the soilsuch as decomposing of recalcitrant material and are key elements in thecarbon and nitrogen cycle. However, VWC measurements are limited bysmall-scale variations, due to geomorphological characteristics and soilproperties, such as the saturated hydraulic conductivity, that occur inthe spatial range of a few tens of meters and in the temporal range of afew days. To address such needs, different methods have been developedover the years.

Existing methods of VWC quantification include conventional gravimetry,wired sensing, remote sensing, and wireless techniques. The conventionalgravimetric method involves collecting a sample of the soil from thefield followed by weighing the sample before and after drying. Thedisadvantages of the gravimetric method are the laborious physicaleffort, the compaction of the soil during transportation and theinaccuracy caused by the water loss due to evaporation. The physicaleffort in moving the soil samples from the field to the laboratory canbe avoided by wired sensing methods that calculate VWC by measuring thedielectric permittivity of soils.

Although a variety of sensors and probes have been developed for in situmeasurement and quantification of VWC, they are limited in the fact thatthey need wire connection to be interfaced with external data loggers orpossible wireless data transmission nodes. For example, existingtechnologies (imaging spectroscopy, field-deployable sensors, andpassive RFIDs) used for constant monitoring of soil properties have thefollowing drawbacks. The wired systems are slow to deploy and are atrisk of being damaged by farm machinery during regular farming activity.To address this limitation, different large scale imaging systemsnamely, hyperspectral imagery and high-resolution satellite imagery havealso been utilized in precision agriculture application. This methodinvolves collecting images of the field and assessing theelectromagnetic spectrum at fine resolution. While these image-basedtechnologies work best for analyzing the spatial variability of cropyield, the main drawback of these methods is that they cannot be usedfor extracting information from beneath the ground level. Therefore,imaging spectroscopy cannot be used for extracting information fromunderneath the soil. Furthermore, field-deployable sensors are oftenwire-based, costly, and power-hungry. Passive RFID technology solvesthese issues through wireless communication between the reader and thesensor. While RFID based technologies are promising due to theirbattery-less operation, they still need electronic components for energyharvesting and sensing. Furthermore, since electronic chips are notbiodegradable, burying them leads to accumulation of e-waste in thesubsoil. Therefore, there is a great need for subsoil wireless sensingtechnologies that can be developed using fully biodegradable materialsto eliminate the need for retrieval after their task has been completedin the field. In addition, to automate the process of planting sensorsin the soil, the sensor needs to be small enough to fit inside a cornplanter so that they can be planted during plowing without additionaleffort.

Therefore, there is an unmet need for a novel approach that enablesconvenient measurement of subsoil moisture across a large field thatwill not be damaged by regular farming activity.

SUMMARY

A batteryless, chipless, sensor is disclosed which includes a substrate,at least two conductive strips positioned on the substrate, and apassivation layer encasing the substrate and the at least two conductivestrips. The conductive strips are adapted to respond to an interrogationsignal from a reader having a first polarization, with a response signalat a second polarization different than the first polarization.

A system of determining soil conditions is also disclosed. The systemincludes one or more ground interrogating devices, each configured toradiate a wireless interrogation signal at a first polarization. Thesystem also includes a plurality of ground-embedded battery-less andchipless sensors. Each such sensor includes a substrate, at least twoconductive strips disposed on the substrate, and a passivation layerencasing the substrate and the at least two conductive strips. Theconductive strips are adapted to receive the interrogation signal fromthe one or more ground interrogating devices, and in response theretoprovide a response signal at a second polarization different than thefirst polarization. The response signal corresponds to a plurality ofsoil variable associated with soil conditions. The system furtherincludes a server configured to receive signals from the one or moreground interrogating devices. Additionally the system includes at leastone input/output device in communication with the server and configuredto provide control signals to the one or more ground interrogatingdevices and to receive data associated with the soil variable associatedwith soil conditions.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 a is an example schematic for a system adapted to determine soilconditions utilizing one or more ground interrogating devices and aplurality of ground-embedded battery-less and chipless sensors.

FIG. 1 b represents schematics of signal propagations from the one ormore ground interrogating devices and the plurality of battery-less andchipless sensors of FIG. 1 a , according to different embodiments of thepresent disclosure.

FIG. 1 c is a lumped parameter schematic of the battery-less andchipless sensor of FIG. 1 a , according to the present disclosure.

FIG. 2 a provide exemplary schematics which show the working principleof the battery-less chipless sensor tag of the present disclosure.

FIG. 2 b is a graph of S₂₁ in dB vs. frequency, where S₂₁ represents theratio of the power received at one port of a Vector Network Analyzer(VNA) coupled to the receiving section of an antenna receiving signalfrom the sensor to the power transmitted from another port of the VNAcoupled to the transmitting section of the antenna.

FIG. 3 is a schematic of a test setup of the sensor, according to thepresent disclosure.

FIGS. 4 a and 4 b are graphs of S₂₁ in dB vs. frequency in Hz of resultsof simulations showing the working of the sensor, wherein FIG. 4 aprovides the results in air, in dry soil, with a passivation layer inair, and with a passivation layer in dry soil; while FIG. 4 b providesthe results with a passivation layer in dry soil and two differentwetness levels resulting in various dielectric constants illustrated asthe calibrated S₂₁ after passing the results through an FFT smoothingfunction to remove the ripples.

FIGS. 5 a and 5 b are schematics which provide manufacturing processesillustrating the fabrication steps in FIG. 5 a and the final sensor formin FIG. 5 b.

FIGS. 6 a and 6 b provide photographs of experimental setup with areader antenna and the sensor in air (FIG. 6 a ) and in soil (FIG. 6 b).

FIG. 7 provides graphs of S₂₁ (calibrated) in dB vs. frequency in GHzfor multi-tag measurements with 45 degree clockwise (CW) andcounterclockwise (CCW) rotations for experiments and simulations.

FIG. 8 provides graphs of S_(21(calibrated)) in dB vs. frequency in GHzfor which illustrate the response obtained when the free space distance(S) is 30 cm between the interrogator and the sensor (result areprovided for wet soil experiments demonstrating shift in the resonantfrequency as a function of volumetric water content (VWC) for L_(tag)=9cm and 10 cm.

FIG. 9 provides plots of frequency vs. water added to soil sample,plotted for L_(tag)=9 and 10 cm when the free space distance between theinterrogator and the sensor is 30, 40, and 50 cm.

FIGS. 10 a and 10 b are plots which show frequency absolute frequencyshift vs. VWC (%) comparing between simulation results and experimentaldata showing frequency and absolute frequency shift as a function of VWCfor (a) L_(tag)=10 cm (b) L_(tag)=9 cm and the calculated values.

FIG. 11 a set of graphs of S_(21(calibrated)) in dB vs. frequency in GHzrepresenting angular dependence vs. frequency for different angles areprovided with 45° providing the best angle.

FIG. 12 is a graph of S₂₁ in dB vs. frequency in Hz for differentrotational angles of the sensor between 0° and 60° with increments of15°.

FIGS. 13 a, 13 b, 13 c, 13 d, and 13 e are photographs of variousembodiments of the sensor, according to the present disclosure incomparison to the size of a U.S. penny.

FIG. 13 f is a graph of S_(21(calibrated)) in dB vs. frequency in Hz forthe different embodiments shown in FIGS. 13 a -13 e.

FIGS. 14 a and 14 b are cross sectional and perspective schematic viewsof the sensor of the present disclosure, respectively, chosen so thatthe resonant frequency is less than 1.3 GHz.

FIG. 14 c is a top view of an actual reduction to practice of the sensoraccording to the present disclosure of the sensor shown in FIGS. 14 aand 14 b.

FIG. 15 a is a schematic of the electrical equivalence of the sensor inan RLC parallel circuit.

FIG. 15 b provides cross-sectional parametric view of the sensor shownin FIG. 15 a of a cross-section ‘A-A’ shown in FIG. 15 b depicting anexploded and schematic view of the sensor with 10 folds.

FIG. 16 provides a flow of the manufacturing process for manufacturingthe sensor tag, according to the present disclosure.

FIG. 17 is a photograph of placement of the sensor of the presentdisclosure into a volume of soil.

FIG. 18 is a complex set of graphs of several frequency responses vs.frequency for different values of VWC.

FIGS. 19 a and 19 b are photographs of deployment of the sensor of thepresent disclosure into ground and placing the interrogator at differentheights.

FIG. 19 c provide graphs in two panels (i and ii) comparing laboratoryresults vs. field results which in one panel (ii) provides response vs.frequency for different measurement heights for each of the threeidentical sensors as shown in FIGS. 14 b and 15 b and in the other panel(i) compares these measurements to the laboratory measurements.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of thepresent disclosure, reference will now be made to the embodimentsillustrated in the drawings, and specific language will be used todescribe the same. It will nevertheless be understood that no limitationof the scope of this disclosure is thereby intended.

In the present disclosure, the term “about” can allow for a degree ofvariability in a value or range, for example, within 10%, within 5%, orwithin 1% of a stated value or of a stated limit of a range.

In the present disclosure, the term “substantially” can allow for adegree of variability in a value or range, for example, within 90%,within 95%, or within 99% of a stated value or of a stated limit of arange.

A novel approach that enables convenient measurement of subsoil moistureacross a large field is presented that will not be damaged by regularfarming activity. To address this need, here, the present disclosurepresents a biodegradable battery-less chipless wireless seed sensor.This approach provides an enhanced environmental-friendly measurementtechnology that can be leveraged for easy deployment over a wide area.The passive chipless technology has previously been demonstrated inwireless sensing applications such as item-tracking, humidity sensing,and gas detection. Battery-less chipless sensors work based on theprinciple of backscattering. The interrogator-reader module includes anRF source which sends an electromagnetic signal to the sensor. Thissignal interacts with the sensor and reflects from its surface. Thereflected signal contains the resonant frequency information of thesensor and can be detected by the reader. The resonant frequency is afunction of the dimensions of the sensor and the dielectric propertiesof its surroundings. Using this property, non-destructive sensing of theenvironment is possible, by coating the sensor with a functionalmaterial sensitive to the environment, e.g., humidity, temperature, andchemical composition in the soil.

Specifically, the sensor can be coated with a biodegradable functionalpolymer that is sensitive to soil parameters such as moisture,temperature, nitrate concentration, and microbial activity. When thesensor is buried underneath the soil, the dielectric constants of thepolymer coating and the soil change as the environmental factors change,leading to a shift in the resonant frequency of the sensor. Thereader/interrogator can deduce the change in the dielectric constants ofthe functional polymeric coating and the soil from the interrogatedresonant frequency and estimate the soil parameter under consideration.

Towards this end, a system with a plurality of passive wireless chiplesssensors is presented. These sensors work based on frequency domain ortime domain approaches using resonating structures that are accessed byone or more remotely positioned interrogator/reader that transmitsenergizing signals to the sensors. This transmitted signal interactswith the resonator and reflects from the surface of the resonator. Thereflected signal contains the encoded information of theresonator/sensor and conditions of its surrounding, as discussed above.Since the passive wireless chipless sensors made of these resonators donot require on-board batteries to power them up or electronic chips tomeasure the parameters of interest, they are inexpensive, powerefficient, integration-friendly, and printable on scalable manufactureplatforms. Since subsoil telemetry requires a power-efficient,inexpensive, high-resolution measurement system to address the need foraround-the-clock surveillance of spatially variable subsoil, theadvantages of passive wireless chipless sensing technologies make theman ideal candidate for subsoil applications.

The interrogator/reader includes an RF source that sends theinterrogation signal to the ground. The signal impinges on the sensorand backscatters from its surface. The backscattered signal is collectedby the interrogator/reader. The sensors include a tag made from metallicmicrostrip antenna structures with a resonant frequency that is afunction of the length of the tag and the effective dielectric constantof the media surrounding the tag. Due to a significant difference indielectric properties between water (80) and dry soil (about 2), anincrease in soil volumetric water content (VWC), results in a largechange in the effective dielectric surrounding the sensor tag whichresults in a change its resonant frequency. The resonant frequency ofthe tags can be wirelessly measured by analyzing the spectrum of thebackscattered signal received by the interrogator/reader antenna. As aresult of this changes, the effective dielectric properties of themedium of interest, e.g., soil, can be extracted from the measuredresonant frequency of the buried sensor tag.

The wireless reading from the sensor tags can be achieved by a singleinterrogator/reader module fixed onto a drone that hovers over thefield, or on a field vehicle that rovers over the surface. Referring toFIG. 1 a , an example of a system 100 of the present disclosure isprovided. As stated above, a variety of different interrogators, e.g., afarm vehicle 102, a drone 104, etc., can be used to transmitinterrogation signal to and receive response signals from one or moresensor tags 106 embedded in the soil 108. The signal paths includeinterrogation signals 110 emitted from the interrogators (i.e., the farmvehicle 102, the drone 104, etc.) and reflected signals 112 from thesensor tags 106. The received response signal, according to oneembodiment is communicated to one or more cloud servers 120 which can beremote or local to the farming operation via a communication link 122which can be a wired or a wireless communication link and is processedto extract the soil parameters. The one or more cloud servers 120 canthen communicate soil health information to I/O devices (e.g., mobilehandheld devices) 124 via a wired or wireless link 126 coupled to theone or more cloud servers 120, thereby forming a complete high spatialand temporal mapping of the VWC in the agricultural field.

A major component of the system of the present disclosure is thechipless batteryless sensor tags 106. These tags are provided anddescribed herein in various geometries namely, hairpin resonators, slotresonators, split ring, and shorted dipoles. The choice of the tagdepends on the reading method and the application. For example,according to one embodiment of the present disclosure, a wirelessinterrogation approach could use a linearly polarized backscatteredsignal. Despite the simplicity of interrogator/reader antenna in such RFbased wireless integration approaches, the transmitter and the receiverare in the same polarization which results in cluttered backscattersignal readings from both the sensor tag 106 and its surroundings. Inother words, when the interrogating RF signal propagates toward thesensor tag 106, it reflects not only from the surface of the tag, butalso from the surrounding materials and any objects in its path. If theimpeding objects do not have the ability to alter the orientation of theincident signal, they reflect the signal in the same orientation as theincident signal. Therefore, interrogator/reader receives the reflectionsfrom the clutter as well from the sensor tag in the same polarization.These undesirable RF reflections, combined with the antenna losses,increase the overall noise floor of the backscattered signal received bythe interrogator/reader, thereby making it difficult to filter thedesired information as shown by S_(21(no tag)), e.g., in iteration i ofFIG. 1 b which provides schematics of the signal propagations accordingto different embodiments. Specifically, FIG. 1 b (iterations i and ii)are schematics which provides a comparison between interrogator/readersof various polarizations amongst different interrogating tags. As can beseen in the panel i of FIG. 1 b , the response from the tag has the samepolarization as the incoming signal from the interrogator/reader (i.e.,the farm vehicle 102, the drone 104, etc., shown in FIG. 1 a ). Theseundesirable reflections from the surrounding environment can be avoidedby using a transmitter and a receiver that operate incross-polarization, i.e., transmitter sends signals in the verticalpolarization and the receiver receives signals in the horizontalpolarization as shown in the panel ii of FIG. 1 where the noise floor,S_(21(no tag)), is below the desired signal level, S_(21(tag)), owing tothe design of the tag which enables a response in a polarization that isdifferent from the incoming interrogator/reader signal. That is in panelii of FIG. 1 b , the reflected signal from the sensor tag 106 is 90°phase-shifted with respect to the incoming (incident) signal emanatingfrom the onboard reader/interrogator (i.e., the farm vehicle 102, thedrone 104, etc., shown in FIG. 1 a ).

A depolarizing sensor tag, as shown in the schematic of the panel ii ofFIG. 1 b , is a chipless device that can convert the vertical polarizedsignals received from the transmitter to both vertically andhorizontally polarized signals. Depolarizing refers to the ability of adevice to convert the polarization of signals that impinge on it. Here,the depolarizing sensor tag converts the polarization of the incomingsignal to horizontal and vertical polarizations. The phrase S₂₁ in FIG.1 b and other places herein refers to the ratio of the power received atone port of a Vector Network Analyzer (VNA) coupled to the receivingsection of the antenna to the power transmitted from another port of theVNA coupled to the transmitting section of the antenna. The signals fromthe reader antennas are vertically polarized which increases thepossibility of vertically polarized reflections from the background.However, reflections from the background are less likely to behorizontally polarized. Therefore, the reflected signals from thechipless tag are less likely to be lost in the vertically polarizedreflections from the background if the reflections from the chipless tagare horizontally polarized. Hence, a depolarizing tag that can receivesignals in the vertical polarization but reflect them back in horizontalpolarization is more likely to be noise-free. Therefore, thehorizontally polarized receiver can pick up more specific horizontallypolarized signals from the sensor tag with less interfering backgroundnoise.

Referring to FIG. 1 c , a lumped parameter schematic of the sensor tagaccording to the present disclosure is provided. The lumped parameterincludes a lumped resistor (R_(rad)), a lumped inductor (L_(meander)),and a lumped capacitor (C_(meander)), all three provided in a parallelfashion.

One such exemplary tag includes a shorted dipole antenna wheninterrogated at 45° with respect to the receiver, as shown in the panelii of FIG. 1 b . In addition to the depolarizing properties, the shorteddipole structure (discussed further below), occupies less area, iscentrosymmetric, and is easy to manufacture using scalable manufacturingtechniques. The resonant frequency of such tags can be obtained usingthe following formula:

$\begin{matrix}{f_{r} = \frac{c}{2L_{tag}\sqrt{\varepsilon_{eff}}}} & (1)\end{matrix}$where f_(r) is the resonant frequency;c is the speed of light;L_(tag) is the length of the shorted dipole antenna; andε_(eff) is the effective dielectric constant surrounding the dipoleantenna. By using this equation, one can roughly estimate the designparameters of the sensor tag (e.g., the sensor tags 106 shown in FIG. 1a ) and its operating resonant frequency. For instance, for a shorteddipole antenna having L_(tag)=10 cm, and a substrate of

${\varepsilon_{r} = {2\left( {\varepsilon_{eff} \approx \frac{\varepsilon_{r} + 1}{2}} \right)}},$f_(r) is approximately calculated to be 1.22 GHz.

Referring to FIG. 2 a , exemplary schematics are provided which show theworking principle of the battery-less chipless sensor tag of the presentdisclosure. FIG. 2 a provides schematics illustrating the shift in theresonant frequency when the sensor is surrounded by air, a passivationlayer, dry soil and wet soil. For a fixed length of the tag, theresonant frequency is a function of the effective dielectric constant(ε_(eff)) in the fringing electric field between the periphery of thedipole antenna and the ground plane of the tag. As illustrated in FIG. 2a , the effective dielectric constant of the as prepared tag without apassivation coating is shown as ε_(eff0) which is a function of thedielectric constant of the substrate ε_(r) and its surrounding air(about 1), resulting in an initial resonant frequency denoted as f_(r) ₀shown in FIG. 2 b which is a graph of S₂₁ in dB vs. frequency. Bypassivating the sensor tag with a polymer coating (with dielectricconstant ε_(r) greater than air (which is about 1), the ε_(eff)increases to ε_(eff) ₁ which decreases the sensor's resonant frequencyto f_(r) ₁ . As discussed with respect to FIG. 1 a , the sensor tags areburied underneath the soil and has a resonant frequency that isdependent on the dielectric constant of the surrounding soil. As themoisture content in the soil increases, the presence of bound water(with dielectric constant of about 80) within the soil increases. Thiscauses an increase in the ε_(eff) leading to a decrease in the resonantfrequency reading from the sensor. By analyzing the changes in theresonant frequency, one can wirelessly retrieve the VWC of the soilsurrounding the sensor. Referring to FIG. 2 b , response vs. frequencyfrom the tags are shown for different conditions (e.g., when the sensoris surrounded by air (f_(r0)), passivation layer (f_(r1)), dry soil(f_(r2)) and wet soil (f_(r3))). As seen in FIG. 2 b , the naturalfrequency of the tag is lower when in contact with wet soil vs. when itis in contact with dry soil. This difference can be used advantageouslyto determine the wetness of the soil. The dashed line in FIG. 2 adepicts the change in the natural frequency as conditions vary.

The design of the shorted dipole mainly depends on the working resonantfrequencies band in which the sensor tag needs to operate. The upperlimit of the band depends on the resonant frequency of the tag in air,which in turn depends on the inductance (L_(tag)) and the dielectricconstant of the substrate (ε_(r(sub))).

For effective wireless reading through soil, the working range of thesensors' resonant frequencies must support good levels of RF signalpenetration into the soil while having a reasonable sensor tag size forpractical field deployment. The depth of penetration of the EM signalsinto a soil depends on its dielectric constant and electricalconductivity. The dielectric constant of soil can range from about 5 toabout 30. The range of radio waves used for ground penetration is lessthan about 2 GHz. According to one embodiment, frequencies between about900 MHz and about 1 GHz are chosen for the band so that the two peakscan act as an electromagnetic identification (ID) that indicates thepresence of the sensor and avoids inadvertent readings. Therefore, atwo-tag configuration is used in the sensor.

The sensor tags of the present disclosure are simulated in CST MICROWAVESTUDIO. The simulations are performed on a sensor tag structure shown inFIG. 3 , which is a schematic of the test setup. In this design, thesensor tags are assembled on to a 10 cm×10 cm×2.54 mm acrylic substrate.Tags of dimensions of 10 cm×1 cm×17.5 μm and 9 cm×1 cm×17.5 μm areplaced on top of the acrylic substrate with a spacing of 4 mm. A groundplane of size 10 cm×10 cm×17.5 um is placed on the back of the acrylicsubstrate. A 2 mm thick layer is placed to passivate the top and bottomof the structure. A 10 cm thick layer of soil is assumed in the frontand a 2 cm thick layer of soil is assumed in the back of the resultingstructure. The material for the tags and ground plane are assigned tocopper and that for the passivation layer to polydimethylsiloxane(PDMS). The S₂₁ calibrated to remove the background noise is obtainedfrom the horizontal component of the Radar Cross Section (RCS) in CSTMICROWAVE STUDIO.

Referring to FIGS. 4 a and 4 b , results of simulations showing theworking of the tags are provided: FIG. 4 a provides the results in air,in dry soil, with a passivation layer in air, and with a passivationlayer in dry soil; while FIG. 4 b provides the results with apassivation layer in dry soil and two different wetness levels resultingin various dielectric constants illustrated as the calibrated S₂₁ afterpassing the results through an FFT smoothing function to remove theripples. When the tag is not passivated and placed in air, f_(r)obtained for L_(tag)=10 cm and L_(tag)=9 cm are 1.0025 GHz 1.1075 GHz,respectively. In the next simulation, the sensor is passivated with aPDMS layer of thickness 2 mm and ε_(r) of 2.2 and is placed in air.Since the passivation layer increases the ε_(eff), there is a 5.23% and6.7% shift in the f_(r) for L_(tag)=10 cm and 9 cm, respectively. Thisincreases to 8% and 8.5% (L_(tag)=10 cm and 9 cm, respectively) causedby the increase in the ε_(eff) when the medium is changed from air todry soil. It should be noted that VWC of dry soil is about 2.55.However, it is lower than the frequency shift for the non-passivatedsensor in dry soil because the former is contributed by materials ofhigher effective dielectric constant. FIG. 4 b shows the shift in f_(r)simulated with a passivated sensor in wet soil. It should be noted thatVWC of the wet soil is about 18.8 when the dielectric constant is about10. The wet soil simulations are done by increasing the ε_(r(soil)).When the ε_(r(soil)) is increased to 5, the shift in f_(r) is 5% and6.5% for L_(tag)=10 cm and 9 cm, respectively. The shift in f_(r)increases to 8.45% and 10% for L_(tag)=10 cm and 9 cm, respectively, asthe ε_(r(soil)) is increased to 10 following the Eq (1).

Two types of tags are developed for the experiments. The first type oftags does not have a passivation layer. This is used in the initialexperiments to demonstrate the working of the sensor in air. The secondtype of tag has a passivation layer. This is used in the second set ofexperiment used for soil measurements. The fabrication steps are shownin FIGS. 5 a and 5 b which provide manufacturing process illustratingthe fabrication steps in FIG. 5 a and the final sensor form in FIG. 5 b. The process begins by providing a substrate (e.g., acrylic).Thereafter, sheets of conducting material (e.g., copper), are depositedon the substrate. Thereafter, material from the upper conducting sheetis removed leaving two strips of metal, resulting a subassembly.Thereafter, a layer of passivation (e.g., PDMS) is placed over thesubassembly, and finally the passivation layer is formed to fit the sizeof the subassembly.

Specifically, the fabrication process shown in FIG. 5 a starts with asubstrate (acrylic, according to one embodiment). Based on thesimulation results and the commercial availability, the thickness ischosen to be about 2.54 mm, without imposing any limitation thereon. Thesubstrate is laser cut to form a 10 cm×10 cm layer (see FIG. 5 a , paneli). Conducting material, e.g. metal, e.g., copper tapes are attached tothe bottom and top as shown in FIG. 5 a , panel ii. Metal is partiallyremoved away using a removal process (e.g., etching, laser cutting,etc.) resulting in tags on one surface, as shown in FIG. 5 a panel iii.For the first set of measurements, 5 different tags are used on the samesubstrate. The tags are of length: 10, 9, 8, 7, and 6 cm. This sensor isused for the first set of measurements in air. For the soilmeasurements, the metal tapes are laser cut on the top surface to formtwo strips of size 10 cm×1 cm and 9 cm×1 cm (see FIG. 5 b and FIG. 5 apanel iii). Both strips as well as the backside of the tag are coatedwith a 2 mm thick passivation layer, e.g., a PDMS layer, followingstandard procedures (FIG. 5 a panel iv). The tags are completelyimmersed in a solution of passivation (e.g., PDMS (1:10,cross-linker/polymer)) and degassed for 30 minutes in a vacuum chamber.The edges of the passivation layer are cured to form the final structure(as shown in FIG. 5 a panel v). This forms the passivation layer thatprotects the tags from electrical shortage. The fabricated sensor isshown in FIG. 5 b.

Two types of tags are developed for the experiments. The first type oftag does not have a passivation layer. This is used in the initialexperiments to demonstrate the working of the sensor in air and drysoil. In this experiment, the ability of multiple tags to operate inconjunction with each other on the same sensor is tested and the maximumdistance at which the tags can be detected in air is measured. In thedry soil measurements, the relation between the resonant frequency andthe depth of the tag in the soil is studied. The second type of tag isused in soil measurements. In this experiment, water is addedprogressively to the sample and the percentage shift in the resonantfrequency is investigated. A commercial sensor is used simultaneously toobtain ground truth values. A comparison is drawn between the frequencyshift and the volumetric water content.

The experimental setup is shown in FIGS. 6 a and 6 b (photographs ofexperimental setup with a reader antenna and the tag in air, FIG. 6 aand in soil FIG. 6 b ). A quad-ridge horn antenna is used as the reader.The vertical ridge is used as the transmitter (Tx) to generate avertically polarized interrogation signal and the horizontal ridge isused as the receiver to read the horizontally polarized signalbackscattered from the sensor. The vertically polarized Tx is connectedto port 1 and the horizontally polarized Rx is connected to port 2 of anAGILENT NETWORK ANALYZER and the S₂₁ is calculated to investigate thefrequency response. FIG. 6 b implements the schematics in FIG. 2 a . Thetray is filled with soil of known volume. Water is added to the trayprogressively and mixed thoroughly to make its distribution uniformthroughout the tray. The experiments are performed by changing both theseparation between the reader and the surface of the soil (S) and thedepth of the tag under the surface of the soil (D). The effect ofbackground is cancelled by calibrating the S₂₁ measurement in all thecases. This is accomplished by measuring the complex S₂₁ without thetag. This provides the S₂₁ of the background and is referred to asS_(21(isolation)). The complex S₂₁ of the tag is also measured and isreferred to as S_(21(tag)). The S₂₁ is calibrated using the followingequation:

$\begin{matrix}{{S_{21}({dB})} = {10\log_{10}{❘{S_{21{({tag})}} - S_{21{({isolation})}}}❘}}} & (2)\end{matrix}$

The calibrated S₂₁ measurement helps in eliminating clutter andfacilitating the detection of the peak of resonance. Moreover, thismeasurement is simpler than RCS measurement because the latter requiresan additional S₂₁ measurement with a structure of known RCS and theformer does not.

The multi-tag experiments are done in two configurations. Initially, thetag is tiled by 45° clockwise in one case and 45° counterclockwise inthe other case. In both cases, the results are expected to be the sameand FIG. 7 , which shows graphs of S_(21(calibrated)) in dB vs.frequency for multi-tag measurements with 45 degree clockwise (CW) andcounterclockwise (CCW) for experiments and simulations, demonstratesthis variation. The f_(r) is the same for both cases at various valuesof L_(tag). As the L_(tag) increases from about 6 cm to about 10 cm withan interval of 1 cm, the f_(r) also shifts as follows: 1.65,1.40,1.23,1.1108 and 0.976 GHz. The peak of the resonance is obtained asfollows as L_(tag) changes from 6 cm to 10 cm: −29.09, −29.33, −28.14,−28.8, and −26.974 dB. The magnitude of the peak reduces as L_(tag)changes from 10 cm to 6 cm. The f_(r) obtained from simulation resultsperformed on CST MICROWAVE STUDIO match exactly with the experiments.The magnitudes of the peaks show the opposite trend. However, thedifference between the peaks and the notches demonstrate that the bestresults are obtained at the lower end of the frequencies. Furthermore,FIG. 7 illustrates a few important aspects in terms of soilapplications: 1) the sensor can operate with more than one tag on thesubstrate without one interfering with the other. The resonantfrequencies are far from each other and do not cause any interference.2) The distance at which the tags are detectable is 2 m. Table 1 alsoshows that this method requires only one antenna. Bistatic measurementson the field are cumbersome because of the requirement that twointerrogating device (e.g., two drones) must surveil the sensors inperfect angular alignment. Monostatic measurements are preferable, butthese measurements are prone to noise when the interrogation signal andthe backscattering signal are in the same polarization.

The soil experiments are done to study the effect of VWC on the resonantfrequency (f_(r)) and the absolute frequency shift (Δf_(r)). The sensoris buried at a depth of 10 cm. FIG. 8 illustrates the response obtainedwhen the free space distance (S) is 30 cm (result are provided for wetsoil experiments demonstrating shift in the resonant frequency as afunction of VWC for L_(tag)=9 cm and 10 cm). In other words, in thisconfiguration, the tags are 9 cm and 10 cm long. The tag with L_(tag)=9cm has an initial frequency of 1.096 GHz. As the water content increasesin the sample, f_(r) reduces and Δf_(r) increases. At 100 ml, 150 ml,and 200 ml, f_(r) are 1.092 GHz, 1.088 GHz, and 1.0747 GHz,respectively. This is expected because ε_(eff) increases with theincrease in the volume of the water added to the sample. A similar trendis observed for the L_(tag)=10 cm. Specifically, at 0 ml, 100 ml, 150ml, and 200 ml, f_(r) are 0.9678 GHz, 0.9643 GHz, 0.9607 GHz, and 0.9505GHz, respectively, the L_(tag)=10 cm.

The experiment is repeated for various values of free space distancesuch as 40 cm and 50 cm. The results are plotted in FIG. 9 (which showplots of frequency vs. water added to the sample, plotted for L_(tag)=9and 10 cm when the free space distance is 30, 40, and 50 cm). From 0 mlto 200 ml, the resonant frequencies exactly follow each other. As theconcentration of water increases beyond 200 ml, the error in the plotsstarts to increase, but is still within acceptable limits. The sametrend is observed for both values of L_(tag): 10 cm and 9 cm.

The mean value and standard deviation (SD) of the frequencies andabsolute frequency shifts are depicted in FIGS. 10 a and 10 b (whichshow frequency absolute frequency shift vs. VWC (%) comparing betweensimulation results and experimental data showing frequency and absolutefrequency shift as a function of VWC for (a) L_(tag)=10 cm (b) L_(tag)=9cm and the calculated values). To obtain accurate measurements of thesample, a commercial sensor (DECAGON 5TE) is used for the ground truthmeasurement. The DECAGON sensor provides the value of the dielectricconstant of its surrounding and is used for estimating the dielectricconstant of the soil. The measured dielectric constant (ε_(eff)) isconverted to VWC using the Topp equation:

VWC = 4.3 × 10⁻⁶ε_(eff)³ − 5.5 × 10⁻⁴ε_(eff)² + 2.92 × 10⁻²ε_(eff) − 5.3 × 10⁻².

The sensitivity of the sensor is calculated by extracting the slope of alinear fit on the plots of f_(r) displayed in FIGS. 10 a and 10 b (whichprovide frequency vs. VWC (in %) for f_(r) and Δf_(r) for L_(tag)=9 cm(FIG. 10 a ) and L_(tag)=9 cm (FIG. 10 b )). In the range of 4% to 22%of VWC, where the best match is obtained, the sensitivity of the sensoris calculated to be −3.92 MHz/VWC % for L_(tag)=10 cm and −4.85 MHz/VWC% for L_(tag)=9 cm in the experimental graphs. The counterparts insimulations are −3.58 MHz/VWC % for L_(tag)=10 cm and −3.85 MHz/VWC %for L_(tag)=9 cm. The sensitivity can be increased by decreasing thethickness of the passivation layer. An important advantage of thisdesign is that the sensitivity of the sensor can be adjusted easily bychanging the thickness of the passivation layer. Increasing thesensitivity also increases the bandwidth (BW) of the reader. This designprovides the highest BW of 135 MHz assuming a maximum observed VWC of27% and a sensitivity of −4.85 MHz/% VWC.

The obtained results reveal that the first order soil simulations can beused to approximate the f_(r), Δf_(r), sensitivity, and BW for a givenvalue of VWC in any soil types by feeding the material properties of thesoil into the simulator. The experiments conducted on a prototype provethe idea of this simplified simulation model has a great potential inthe sensor development for agricultural applications.

By defining the noise floor at −70 dB, the tag angle range foracceptable discrimination between the incident signal and thebackscattered signal is identified as 15° to 75° (45° being the bestcase) in the first quadrant. Referring to FIG. 11 , angular dependencevs. frequency is shown for different angles with 45° providing the bestangle. FIG. 11 (which is a graph of S_(21(calibrated)) in dB vs.frequency in HZ) shows the results for 0° to 45° rotation in 15°increments. FIG. 12 (which is a graph of S₂₁ in dB vs. frequency in Hzfor different rotational angles of the tag between 0° and 60° withincrements of 15°) shows that the sensor is centrosymmetric. Therefore,one can extrapolate the result to the entire first quadrant (0° to 90°).

If the reader is tilted, this angle can be increased to 60° relative tothe horizontal plane, as shown in FIG. 12 . Assuming the minimumelevation of the drone to be 1 m and the maximum line-of-sight readdistance to be 2 m, the angular range, in this case, is 0° to 60°relative to the horizontal plane (i.e., ground level).

According to one embodiment of the present disclosure, in order tominiaturize the sensors, there are two important factors to beconsidered. One, the length of the sensor is inversely proportional tothe resonant frequency. Two, the depth of penetration reduces with anincrease in the operating frequency. For reasonable ground penetrationof RF signals, the resonant frequency needs to be less than 2 GHz,according to one embodiment. However, direct scaling of the resonatorwould increase the resonant frequency due to the inverse relationshipbetween the resonant frequency and the length of the resonator. Sincedirect scaling cannot be used for miniaturization, a meandered linedesign can be implemented for redesigning the resonators, as provided inFIGS. 13 a-13 e (which are photographs of various embodiments of thesensor in comparison to the size of a U.S. penny). As shown in FIG. 13 f, when the number of folds in the meander structure increases, theresonant frequency of the resonator reduces. For instance, when thenumber of shorted dipoles is increased from 2 to 6, the resonantfrequency is reduced from 2.9 GHz to 1.6 GHz. Using a meandered linedesign, the size of the resonator is miniaturized to about 2 cm×about 2cm thereby achieving a 25× area reduction; different embodiments areshown in FIGS. 13 a, 13 b, 13 c, 13 d, and 13 e . Results of responsevs. frequency for each of these embodiments are shown in FIG. 13 f(which is a graph of S_(21(calibrated)) in dB vs. frequency in Hz) forthe different embodiments shown in FIGS. 13 a -13 e. As shown in FIGS.13 a-13 e and correspondingly FIG. 13 f , when the number of folds inthe meander structure increases, the resonant frequency of the resonatorreduces (see FIG. 13 f ). For instance, when the number of shorteddipoles is increased from 2 to 6, the resonant frequency is reduced from2.9 GHz to 1.6 GHz. A meandered line design that includes 10 folds, asshown in FIGS. 14 a and 14 b which are cross sectional and perspectiveschematic views of the sensor, respectively, is chosen so that theresonant frequency is less than 1.3 GHz. A top view of an actualreduction to practice of the sensor shown in FIGS. 14 a and 14 b is alsoshown in FIG. 14 c . The sensor for respective size comparison to a UScoin (quarter) is shown in FIG. 14 c . Using the meandered line design,the size of the resonator is miniaturized to about 2 cm×about 2 cmthereby achieving about a 25× area reduction (see FIG. 14 c ).

The electrical equivalence of the device is an RLC parallel circuit, asshown in FIGS. 15 a and 15 b which are cross-sectional parametric view(FIG. 15 a ) of a cross-section ‘A-A’ shown in FIG. 15 b depicting anexploded and schematic view of the sensor with 10 folds. The equationsgoverning the values of inductance, capacitance, and resistance isprovided in (3-5) below:

$\begin{matrix}{{L_{meander} = {{0.2}L\left\{ {\left\lbrack {{1.4}813{\log\left( \frac{2L}{a} \right)}} \right\rbrack^{{1.0}12} - {{0.6}188}} \right\}{µH}}},} & (3)\end{matrix}$ $\begin{matrix}{{C_{mea{nder}} = \frac{C_{bend}}{N}},} & (4)\end{matrix}$ $\begin{matrix}{{R_{rad} = {3{4.1}5\left( {2\pi\frac{\left( {L - {2wN}} \right)}{\lambda}} \right)^{1.8}}},} & (5)\end{matrix}$where N is the number of bends,L is the total length of the meander structure,w is the gap between the bends,a is the radius of the trace,C_(bend) is the capacitance between adjacent bends, andλ is the wavelength. Accordingly, the resonant frequency isapproximately proportional to the inverse square root of L_(meander) andC_(meander). Therefore, the resonant frequency can be reduced byincreasing L_(meander). Since L_(meander) increases as a function of thenumber of bends in the structure, the resonant frequency can be reducedby increasing the number of meanders. It is to be noted that theincrease in L_(meander) is more dominant than the decrease inC_(meander) leading to reduction in the resonant frequency as the netresult. This approach helps in obtaining a 25× area reduction whileretaining the resonant frequency within the range that supports goodground penetration. Sensors of this dimension can be easily distributedin the soil up to a depth of 10 cm with the help of a standardagricultural planter. Since corn seeds are typically planted at a depthof 5 cm, the sensor can be safely read.

A manufacturing process according to one embodiment is now provided withreference to FIG. 16 (which provides a flow of the manufacturing processof the sensor tag, according to the present disclosure). Wirelesspassive sensing tags are manufactured through standard scalablemanufacturing techniques as shown in FIG. 16 which allow for asustainable production of biodegradable devices. Clear solid substratesare manufactured using an ULTIMAKER 3D printer with polylactic acid(PLA). PLA is a commercially available thermoplastic which is awell-known biodegradable material, widely used in additive manufacturingapplications due to its low melting temperature, low cost, andbiodegradability. Despite being a biodegradable polymer, PLA is a robustmaterial, which is insoluble in water, capable of performing whileburied in the soil for months. The passivating layer covering themeandered structure can be similarly printed on top of the original PLAsubstrate by taking advantage of the fact that PLA does not need a hotprinting platform in order to yield proper structures.

The conductive meandered structures that make up the resonator on thesensor of the present disclosure, in particular as discussed herein withrespect to FIGS. 14 a, 14 b, 14 c, 15 a, and 15 b , are made usingbiodegradable conductive zinc tape and a high-power laser source.Interest in the use of zinc as an alternative to traditional conductivematerials has increased in recent years due to zinc's properties as ahighly conductive, biodegradable, and biocompatible material. Themeandered structure was cut using a UNIVERSAL LASER SYSTEM PLS6MW byengaging the material with a fiber laser. Through this method,conductive traces can be created at high resolution without the need ofhazardous chemicals and expensive, labor-intensive techniques. Due tothe efficiency of laser cutting, thousands of structures can be preparedin a matter of hours.

The sensors of the present disclosure, in particular as discussed hereinwith respect to FIGS. 14 a, 14 b, 14 c, 15 a, and 15 b , are testedusing a portable reader setup. The reader comprises of a portable VNAconnected to a portable dual-polarized log periodic antenna. The sensorsare placed at a depth of 5 cm in a soil sample, as shown in FIG. 17which is a photograph of placement of the sensor of the presentdisclosure into a volume of soil. The VWC is changed by adding water tothe soil sample and manually mixing the sample until the commercial VWCsensor shows a uniform distribution of water in the sample. Thevertically polarized ridge of antenna transmits the signals to thesensor buried in the soil. The backscattered signals are collected bythe horizontally polarized ridge of the antenna. The results are shownin FIG. 18 which is a complex graph of several frequency responses vs.frequency for different values of VWC. When the soil is dry (VWC=3.67%)the resonant frequency is 1.02 GHz. As the soil VWC is increased to5.15%, 6%, 11.7% and 17.7%, the resonant frequency shifts to 0.97 GHz,0.91 GHz, 0.85 GHz, and 0.79 GHz respectively.

In order to test the working of the sensor in a practical scenario,several experiments were performed in an agricultural field, as shown inFIGS. 19 a and 19 b which are photographs of deployment of the sensor ofthe present disclosure (e.g., see FIG. 15 b ) with sensing/interrogationunits disposed at different heights from the sensor. The antenna ismounted onto a height-adjustable holder and placed in the field. Threeidentical sensors as shown in FIGS. 14 b and 15 b are deployed in thefield to confirm the repeatability of the measurements. The sensors aretested at heights 10 cm and 40 cm, as shown in FIG. 19 b . The testsperformed on different days demonstrate variation in the fieldconditions leading to different values of VWC. A commercial sensor isused to obtain VWC values for comparison. The measurements recorded bythe portable reader matches with the values obtained under laboratoryconditions, as shown in FIG. 19 c which are graphs provided in twopanels (i and ii) comparing laboratory results vs. field results whichin one panel (ii) provides response vs. frequency for differentmeasurement heights for each of the three sensors and in another panel(i) compares these measurements to the laboratory measurements.Moreover, the readings obtained from all the three sensors show minimalvariation, hence validate the repeatability of the sensor manufacturingprocess.

This technique can be adapted for accurately measuring other types ofsoil parameters by simply changing the functional polymer films on thesensor that are sensitive to the measured parameters such astemperature, nitrate concentration, and microbial activity detection.

Those having ordinary skill in the art will recognize that numerousmodifications can be made to the specific implementations describedabove. The implementations should not be limited to the particularlimitations described. Other implementations may be possible.

The invention claimed is:
 1. A batteryless, chipless, sensor,comprising: a substrate; at least two conductive strips coupled to eachother and disposed in a meandered profile on the substrate; and apassivation layer encasing the substrate and the at least two conductivestrips, wherein the conductive strips are adapted to respond to aninterrogation signal from a reader having a first polarization, with aresponse signal at a second polarization different than the firstpolarization, wherein the lumped parameters are defined based on:$\begin{matrix}{L_{meander} = {{0.2}L\left\{ {\left\lbrack {{1.4}813{\log\left( \frac{2L}{a} \right)}} \right\rbrack^{{1.0}12} - {{0.6}188}} \right\}{µH}}} \\{C_{mea{nder}} = \frac{C_{bend}}{N}}\end{matrix}$ where N is the number of bends in the meandered profile, Lis the total length of the meander profile, α is the radius of thetrace, and C_(bend) is the capacitance between adjacent bends.
 2. Thesensor of claim 1, wherein the conductive strips are made of metal. 3.The sensor of claim 2, wherein the metal is zinc.
 4. The sensor of claim1, wherein the substrate is made of acrylic.
 5. The sensor of claim 1,wherein the at least two conductive strips are about 6 cm to about 10cm.
 6. The sensor of claim 1, wherein the substrate is made ofpolylactic acid (PLA).
 7. The sensor of claim 6, wherein the conductivestrips are made of zinc.
 8. The sensor of claim 1, wherein theresistance of the sensor is determined based on:$R_{rad} = {3{4.1}5\left( {2\pi\frac{\left( {L - {2wN}} \right)}{\lambda}} \right)^{1.8}}$where w is the gap between the bends, and λ is the wavelength.
 9. Thesensor of claim 1, wherein N is between 1 and
 10. 10. A system ofdetermining soil conditions, comprising: one or more groundinterrogating devices, each configured to radiate a wirelessinterrogating signal at a first polarization; a plurality ofground-embedded battery-less and chipless sensors, each comprising: asubstrate, at least two conductive strips coupled to each other anddisposed in a meandered profile on the substrate, and a passivationlayer encasing the substrate and the at least two conductive strips,wherein the conductive strips are adapted to receive the interrogationsignal from the one or more ground interrogating devices, and inresponse thereto provide a response signal at a second polarizationdifferent than the first polarization; wherein the response signalcorresponds to a plurality of soil variable associated with soilconditions; a server configured to receive signals from the one or moreground interrogating devices; and at least one input/output device incommunication with the server and configured to provide control signalsto the one or more ground interrogating devices and to receive dataassociated with the soil variable associated with soil conditions,wherein the lumped parameters are defined based on: $\begin{matrix}{L_{meander} = {{0.2}L\left\{ {\left\lbrack {{1.4}813{\log\left( \frac{2L}{a} \right)}} \right\rbrack^{{1.0}12} - {{0.6}188}} \right\}{µH}}} \\{C_{mea{nder}} = \frac{C_{bend}}{N}}\end{matrix}$ where N is the number of bends in the meandered profile, Lis the total length of the meander profile, a is the radius of thetrace, and C_(bend) is the capacitance between adjacent bends.
 11. Thesystem of claim 10, wherein the conductive strips are made of metal. 12.The system of claim 11, wherein the metal is zinc.
 13. The system ofclaim 10, wherein the substrate is made of acrylic.
 14. The system ofclaim 10, wherein the at least two conductive strips are about 6 cm toabout 10 cm.
 15. The system of claim 10, wherein the substrate is madeof polylactic acid (PLA).
 16. The system of claim 15, wherein theconductive strips are made of zinc.
 17. The system of claim 10, whereinthe resistance of the sensor is determined based on:$R_{rad} = {3{4.1}5\left( {2\pi\frac{\left( {L - {2wN}} \right)}{\lambda}} \right)^{1.8}}$where w is the gap between the bends, and λ is the wavelength.
 18. Thesystem of claim 10, wherein N is between 1 and
 10. 19. The sensor ofclaim 1, wherein the passivation layer is a polymer coating.
 20. Thesystem of claim 10, wherein the passivation layer is a polymer coating.